In information processing systems such as personal computers, a liquid crystal display (hereinafter referred to as a LCD) is so far known as a display for displaying images such as characters and graphics. There are many kinds of LCDs, but in recent years there is extensively employed an active matrix drive LCD using a switching element such as a thin film transistor (TFT) which is capable of reliably controlling a picture cell density and suitable to the display of fast-moving animation and color images. In the LCD of the TFT type, a plurality of pairs of spaced electrodes is provided, and a plurality of TFTs connected with one another are installed on one of the spaced substrate in the form of a matrix. Also, on the transparent substrate there is installed a plurality of gate lines for turning on TFTs for each row and a plurality of data lines for applying a voltage to the liquid crystal through TFT. Also, a transparent common electrode is formed over the entire surface of the other substrate opposed to the one substrate on which the TFTs are installed, and the liquid crystal is interposed between the spaced substrate.
A TFT active-matrix driven drive circuit for driving a LCD displays an image by turning on each of the switching element rows in sequence by applying a voltage to the gate line, and by applying a voltage, which corresponds in magnitude to the degradation of each pixel corresponding to the switching element row turned on, to the liquid crystal through each data line. During the time the switching element is turned on, the light transmittance of the liquid crystal changes according to the magnitude of the voltage applied through the data line, an electric charge is accumulated in the capacitor of the liquid crystal, and, after the switching element has been turned off, the state that the light transmittance was changed is held by the accumulated charge.
Also, the gate line contains a resistor and a capacitor, and, on the one hand, it takes time between the time that the drive circuit applies a voltage to the gate line and the time that the voltage level of the gate line becomes large enough for turning the switching element. On the other hand, the light transmittance of the liquid crystal changes according to the magnitude of the charge stored in the capacitor of the liquid crystal, but the magnitude of this charge depends upon the period during which the switching element is turned on, and the capacitance part itself of the liquid crystal changes according to the space between a pair of substrates. Therefore, the application of voltage to the gate line and the timing (hereinafter referred to as gate timing) that the application of voltage to the gate line is stopped are determined so that a constant picture quality can be obtained, taking into consideration the capacitor of the gate line of a LCD to be driven and the capacity of the liquid crystal. The drive circuit is designed so that the gate line is turned on and off at the determined gate timing.
However, on the one hand, for the capacity of the gate line of the LCD or the liquid crystal capacity, there are variations for each lot because of the production errors of the LCD. On the other hand, since the drive circuit turns the gate line on and off at the gate timing determined when designed, a disturbance in the displayed image occurs according to characteristics such as the capacity of the gate line of the LCD or the liquid crystal capacity, and therefore there was the problem that a constant picture quality could not be obtained.
Also, when an information processing system such as a personal computer displays an image on a LCD unit comprising a LCD and a drive circuit, the information processing system transmits to the drive circuit of the LCD unit an image data signal indicative of the degradation of each pixel of the image to be displayed, a horizontal synchronous signal, a vertical synchronous signal, and a dot clock signal for fetching data for each pixel from the image data signal. Also, the drive circuit decides the incoming of on-off timing of a predetermined gate line by counting the dot clock signals with the pulse timing of the horizontal synchronous signal as a reference.
However, the frequency of the dot clock signal output by the information processing system is different, depending on the kind of information processing system. Since the drive circuit is designed on the assumption that a dot clock signal of a predetermined constant frequency is input, the on-off timing of the gate line will change if the frequency of the dot clock signal changes. As a result, there was the problem that there could occur disturbance in the displayed image. Also, in order to prevent such disturbance in the displayed image, it was necessary to limit the kind of information processing system that could be connected to the LCD.
Further, in some of the information processing systems, the frequency of the dot clock signal can be changed by software. In such information processing systems, there is the possibility that the frequency of the dot clock signal is changed at an arbitrary timing, depending upon the software that is executed by the information processing system. Therefore, an image cannot be displayed with a constant picture quality with respect to such software.